Wireless communication networks provide a plurality of different services to users. A user typically has a user equipment, UE, e.g. a mobile phone, a laptop, Personal Digital Assistant, PDA or any other type of terminal be which the user makes use of one or more services offered by the wireless communication network.
The wireless communication network may be based on a variety of different technologies both with regards the Radio Access Network, RAN, and also for the Core Network. One example of such a technology is Long Term Evolution, LTE. LTE uses Orthogonal Frequency-Division Multiplexing, OFDM, in the downlink and Discrete Fourier Transform, DFT-spread OFDM in the uplink. The basic LTE physical communication resources can thus be seen as a time-frequency grid, as illustrated in the example in FIG. 1, where each resource element corresponds to one subcarrier during one OFDM symbol interval (on a particular antenna port). LTE provides sophisticated mobility capabilities for a UE to seamlessly move within the network, as well as detect the presence of a network.
The mobility is based on a continuous search for the presence certain signals of the LTE system. A logical cell in LTE is, currently, defined by the presence of a primary synch signal, PSS, a secondary synch signal, SSS, a physical broadcast channel, PBCH, and a cell-specific reference signal, CRS.
Prom the PSS, a UE may determine a slot timing, ID within a Physical Cell ID, PCI, group. From the SSS, a UE may next determine a radio frame timing, the PCI, a cyclic prefix length, and if Time Division Duplex, TDD, or Frequency Division Duplex, FDD, is applied. From the PCI, a UE may determine shifts used for the CRS, and thus the resource element positions within a physical resource block, PRB, that are used for the first port of the CRS. This allows the UE to decode the PBCH, which provides more complete information of the system configuration, including the system bandwidth.
Since the system bandwidth is unknown to the UE before decoding the BCH, the PSS, SSS, and BCH are only transmitted on a minimum supported system bandwidth, corresponding to the centre 6 PRBs of the system bandwidth.
When a UE has found (and registered to) a system, the UE will continuously perform mobility measurements on neighbouring cells, which involves searching for the presence of neighbour-cells' PSS/SSS, and measuring reference signal received power, RSRP, and reference signal received quality, RSRQ, of any CRS associated with a detected PSS/SSS. Note that these measurements will only be performed assuming the minimum system bandwidth in the neighbouring cell, since the actual system bandwidth in the neighbour-cell is unknown to the UE.
The cell-specific reference signal, also known as the common reference signal, is broadcasted periodically by LTE systems to provide a UE the ability to measure the channel used for certain downlink transmissions. The CRS is, for example, used to demodulate the PBCH, but also for demodulation of the physical downlink shared channel, PDSCH, for, for example, transmission modes 1-4, which are the transmission modes that are primarily used for communication to any LTE Release-8 and Release-9 UE, that is the 3rd Generation Partnership Project, 3GPP technical specification for LTE release 8 and 9 respectively. For these transmission modes, the CRS are also utilized for the purpose of channel state information, CSI, measurements which are reported to the network for improved link adaptation and multiple-input multiple-output, MIMO, downlink processing.
The CRS position in the time/frequency grid for a PRB pair in case of 1, 2 and 4 transmitting antennas is illustrated in FIGS. 2a-2c. Between cells, the RS may be shifted in frequency domain.
The different antenna ports of the CRS are thus mapped to different sets of resource elements in the grid. Moreover, for all resource elements assigned to a CRS port, the corresponding resource elements will be muted (zero-power) on all other antenna ports. The overhead of the CRS thus increases with increasing number of Tx antenna ports (8, 16, and 24 resource elements per PRB pair, for 1, 2 and 4 antennas respectively).
LTE is however evolving to reduce the dependence on CRS as to allow for more flexible network deployments, where the downlink transmissions control are not constrained to be transmitted using the same antenna setup (e.g., the same transmission point, TP), as the CRS.
Transmission mode 9, which was introduced in 3GPP Release-10, and some legacy transmission modes, are demodulated without resorting to the CRS. Instead, a UE-specific demodulation reference signal, DMRS, is signalled, along with the data transmission. The DMRS is pre-coded in the same way as the data transmission, and thus allows the UE to estimate the effective channel comprising both the MIMO processing and the electromagnetic propagation channel. To provide the network with CSI, a separate CSI reference signal, CSI-RS, is transmitted which the UE can utilize for that purpose. UE-specific DMRS provides great flexibility for the network in dynamically adjusting downlink transmissions.
For 3GPP Release-11 an enhanced physical downlink control channel, ePDCCH, is being specified, which may also be decoded without any aid of the CRS. Hence, with transmission mode 9 and ePDCCH, both control and data signalling can be flexibly transmitted to a UE independently of the CRS.
A problem with the existing solutions is that the CRS causes substantial overhead in the downlink transmissions, which was motivated in legacy LTE releases by increased performance for CRS based control channels and transmission modes. However, for DMRS (UE specific RS) based transmission modes the overhead of CRS does not provide any increased performance, and the unused CRS will just cause additional overhead